The art of spectral sensitization is very important and essential for production of silver halide photographic materials which have high sensitivity and stability. Previously, a wide variety of spectral sensitizers have been developed, and there have also been technical development in the use of these spectral sensitizers, for example, the development of supersensitization combined therewith, methods of adding thereof, and so on.
Sensitizing dyes which have been known to be used for spectral sensitization include, for example, cyanine dyes, merocyanine dyes and rhodacyanine dyes. These spectral sensitizers are used independently or in combination (e.g., for the purpose of supersensitization).
Sensitizing dyes used for photographic materials are not only required to ensure high spectral sensitivities to photographic materials, but also should not cause an increase in fog, should have excellent exposure characteristics (e.g., latent-image stability, reciprocity law characteristic, temperature and humidity dependence upon exposure), should bring about slight changes in sensitivity, gradation and fog generation upon storage prior to exposure, should not remain in a photographic material after photographic processing, and so forth.
In particular , ensuring high spectral sensitivity and high storage stability to photographic materials is an essential requirement of spectral sensitizers, and therefore, numerous efforts have so far been made to fulfill these requirements. The results thereof are described, e.g., in JP-A-60-202436 (The term "JP-A" as used herein means an "unexamined published Japanese patent application"), JP-A-60-220339, JP-A-60-225147, JP-A-61-123834, JP-A-62-87953, JP-A-63-264743, JP-A-01-15534, JP-A-01-177533, JP-A-01-198743, JP-A-01-216342, JP-A-02-42, JP-B-60-57583 (The term "JP-B" as used herein means an "examined Japanese patent publication"), U.S. Pat. No. 4,618,570, and so on. However, the sensitivity and storage stability achieved were not fully satisfactory.
On the other hand, another important technique for silver halide photographic materials is to add various kinds of dyes with the intention of heightening sharpness and color separability.
As for the dyes whose methine chain is substituted by a bridge group at the 2- and 4-positions and further by an alkyl group at the 3-position, 2,2-dimethylpropylene-bridged dye (Dye A illustrated below) is known in J. Am. Chem. Soc., volume 109, pages 5808-5813 (1987), and 2-methylpropylenebridged dye (Dye B illustrated below) in UKr. Khim. Zh., volume 40, number 10, pages 1166 to 1173 (1974).
However, the above-cited dyes have never been examined under the condition of being incorporated in a silver halide emulsion, and their photographic properties in silver halide photographic materials have not been known at all. ##STR2##
On the other hand, descriptions of supersensitization can be found in Photographic Science and Engineering, volume 13, pages 13 to 17 (1969), ibid., volume 18, pages 418 to 430 (1974), T. H. James The Theory of the Photographic Process, 4th edition, page 259 (1977), and so on. It is known that high sensitivity is feasible by properly choosing a sensitizing dye and a supersensitizer therefor.